1. Field of the Invention
The present invention relates to an electric motor having a structure for reducing cogging torque.
2. Description of the Related Art
Generally, in an electric motor having an SPM-type (surface permanent magnet) rotor wherein a plurality of permanent magnets are attached to a surface of a rotor core result in cogging torque due to magnetic attractive force between the permanent magnet and a tooth of a stator core becomes larger. Since the cogging torque may cause a rotational unevenness of the electric motor, the cogging torque needs to be lowered.
As one measure for lowering the cogging torque, a surface of the permanent magnet may be formed as a curved surface (see FIG. 9). However, even when the magnet shape is designed so as to lower the cogging torque, the desired (i.e., low) cogging torque cannot be obtained unless the permanent magnet is accurately positioned and attached to a predetermined position of the rotor core.
Therefore, in order to lower the cogging torque, it is necessary that the permanent magnet be accurately positioned and attached to a predetermined position of the rotor core. As a related art, a technique for forming a groove for positioning a magnet on a surface of a rotor, and a technique for aligning a plurality of permanent magnets, each having a trapezoidal shape, in a circumferential direction of a rotor so that the neighboring magnets are oriented to the opposite directions are described in JP 2007-006621 A.
Generally, a permanent magnet is a sintered body, and thus it is difficult to manufacture a permanent magnet with high dimensional accuracy. Therefore, each permanent magnet is adhered to a rotor while having a certain degree of positional error (gap) or allowance. In this regard, in a synchronous electric motor wherein the number of slots of a stator cores is “s” and the number of permanent magnets adhered to a rotor core is “2p” (i.e., “p” pairs of magnets are adhered), when a lowest common multiple (LCM) of “s” and “2p” is an odd multiple of “p” (i.e., the LCM is indivisible by “2p”), the cogging torque is generated by the permanent magnet which is misaligned with an ideal magnet center position, as explained below.
FIG. 8 is a schematic view of an iron core 100 constituting a conventional rotor. Iron core 100 has a plurality of protrusions 102 for positioning permanent magnets. Each protrusion 102 is a ridge-shaped portion extending straightly and parallel to an axial direction of the core. As shown in FIG. 9, between each protrusion 102, a permanent magnet 104 having a generally rectangular parallelepiped shape is adhered the core so as to constitute a rotor 106. In this case, the number of protrusions 102 is sixteen, and the number of permanent magnets 104 is also sixteen (i.e., eight pairs of magnets are provided).
In light of dimensional tolerance of each permanent magnet 104 and adhering the permanent magnet, as shown in FIG. 10, distance “d” between each protrusion 102 of iron core 100 is larger than width “w” of each permanent magnet 104. Therefore, when the permanent magnets are adhered to iron core 100, as shown in FIG. 10, neighboring permanent magnets 104 on both sides of a protrusion 102a are attracted to each other in the direction of an arrow by magnetic attractive force thereof and are positioned with substantially no gap between each magnet and protrusion 102a, and neighboring permanent magnets 104 on both sides of a protrusion 102b are positioned with a gap between each magnet and protrusion 102b. In other words, each permanent magnet is offset or deviated from an ideal center position as indicated by a dotted line 108.
When rotor 106 is positioned inside a stator core 112 having nine slots 110 so as to constitute a synchronous electric motor 114 as shown in FIG. 11, an LCM of the number of the slots (s=9) and the number of pairs of poles (p=8) is an odd multiple of “p” (i.e., the LCM=72). FIG. 12 shows the cogging torque generated a number of times per revolution of the electric motor, wherein the number corresponds to the LCM of “s” and “p.” In FIG. 12, a dashed line 116 indicates the cogging torque when each permanent magnet is positioned at ideal center position 108 (see FIG. 10), and a solid line 118 indicates the cogging torque when the permanent magnet is deviated from the ideal center position as shown in FIG. 10. In this regard, solid line 118 includes a plurality of (five in the drawing) graphs, wherein the cogging torque is increased as an amount of deviation of the magnet from the ideal center position is increased.
As can be seen from FIG. 12, in the synchronous electric motor having “s” slots and “2p” poles, when the LCM of “s” and “p” is the odd multiple of “p,” the neighboring permanent magnets are displaced closer to each other by the magnetic attractive force so as to form a pair of magnets, whereby each permanent magnet is offset or deviated from the ideal position. Therefore, relatively large cogging torque is generated and the performance of the motor may be deteriorated. In addition, in JP 2007-006621 A, the shape of each permanent magnet is a trapezoid which is more expensive than a permanent magnet having a simple rectangular parallelepiped.